Densely packed incoherent wavelength-division-multiplexed (WDM) lightwave communications systems have been attracting increasing interest as an alternative system architecture to coherent (heterodyne) systems. A key component in realizing an incoherent WDM lightwave communications system is the lightwave receiver providing filtering, amplification and detection of the lightwave signals. In realizing the lightwave receiver and accomodating the dense packing of the wavelength-division-multiplexed lightwave signals, attention has been focussed primarily on the lightwave or optical filter to the detriment of the lightwave amplifiers and detectors.
Addressing the problem of lightwave filtering, considerable research activity has focussed on the use of distributed feedback (DFB) semiconductor laser structures as in-line filtering devices. Such distributed feedback (DFB) semiconductor laser structures have been operated with a bias set below the threshold lasing current, I.sub.th. Filtering provided by below threshold operation in DFB lasers has been reported over a tuning range of 8 .ANG..
The "in-line" nature of the device is necessitated by the requirement for subsequent amplification and detection of the received lightwave signals. Moreover, as in-line elements, the laser structures are transmissive. By its very nature and use, the in-line element accepts a lightwave signal as input (usually through one facet or end face) and performs the desired function such as filtering on the signal to provide a lightwave signal as output (usually through another facet opposite the one facet). In other words, the in-line element as realized by the DFB semiconductor laser structure accepts lightwave signals at one end and processes the signals internally so that they emerge as output at an opposite end. In so doing, the lightwave signals appear to traverse the in-line element.
Other in-line elements mentioned above for accomplishing lightwave signal reception are amplifiers and detectors. Travelling wave structures (one pass) and Fabry-Perot structures (multi-pass) have been proposed for the optical amplification elements. Generally, a diode structure such as a photodiode or avalanche photodetector is employed to detect lightwave signal activity and, as a result, translate lightwave signal activity into corresponding electrical signal activity.
In an effort to reduce the overall number of elements in a lightwave receiver, semiconductor laser structures have been employed to act as dual-function in-line devices. For example, in Elect. Lett., Vol 20, No. 19, pp. 794-5, a buried heterostructure laser operated with an applied forward bias at a point below threshold (the lasing current threshold, I.sub.th) exhibited simultaneous lightwave amplification and detection. The buried heterostructure laser was connected in a bus configuration as a regenerator or repeater so that lightwave signals received by the laser traversed the laser and were emitted for further transmission on the bus configuration. In the intended use described in the cited reference, the dual-function in-line element (buried heterostructure laser) was, of necessity, a transmissive element.